Abstract An important goal of systems neuroscience is to understand how the brain effectively uses sensory information across constantly shifting and diverse contexts. Sensory encoding and perception are strongly modulated by internal cognitive variables, most notably by attention and arousal. Despite their key role, how they work together to shape cortical network activity remains largely unknown. This work will characterize how visual cortical networks encode stimuli and influence perception when influenced by attention and arousal, both independently and synergistically. Arousal and attention improve visual encoding accuracy by modulating cortical networks. Arousal improves visual encoding nonspecifically, whereas attention only improves visual encoding for stimuli within an attended region of the field of view. These modulatory systems improve perception of near- threshold stimuli, in part through enhancing the response magnitude generated by neurons encoding these stimuli. It is possible that these cognitive factors would combine, resulting in particularly strong increases in stimulus-evoked neural responses and perception. However, attention works through both enhancing the representation of the attended stimuli and concomitantly suppressing unattended, distractor stimuli. Therefore, attention and arousal may negatively interact because arousal is nonspecific, increasing responses to distractors. Gaining insights into the nature of this interaction will require a novel behavioral paradigm, in which both attention and arousal can be modulated separately or together. To this end, I have developed a primate locomotion system that enables monkeys to walk while head-restrained and performing cued-attention visual tasks. Spatial attention in the primate visual system is a well-studied, robust phenomenon that makes use of machinery shared with that of humans. Furthermore, my preliminary data strongly suggests that primates, like mice, exhibit locomotion-induced increases in cortical arousal, as indicated by reduced low-frequency oscillations and increased pupil diameter and neural firing rates. Therefore, this novel behavioral paradigm represents an ideal way to characterize the interaction between attention and arousal in visual cortical networks. In Aim 1, I will determine how locomotion influences the state of visual cortical networks by recording the activity of large populations of neurons in area V4 during stationary and locomotion conditions in the absence of visual stimuli. In Aim 2, I will characterize how locomotion-induced arousal modulates how this network encodes visual stimuli and the impact it has on stimulus perception by recording neural activity as monkeys perform a delayed match- to-sample orientation-change detection task in stationary and locomotion conditions. In Aim 3, I will determine the impact on perception and visual encoding of combined locomotion-induced arousal and attentional modulation by recording neural activity as monkeys perform a cued-attention version of the orientation-change detection task. This project will reveal how attentional and arousal modulations combine in shaping cortical activity and behavior.